A generalized Richards equation for surface/subsurface flow modelling

An approach for modelling surface/subsurface flow and transport in a fully integrated way is presented. The diffusive wave approximation used to model runoff is formulated as a nonlinear Darcy law and coupled with the Richards equation for variably saturated flow. The water dynamics in the three physical domains, land surface, vadose zone and saturated zone, is described through a single Richards type equation with domain-dependent parameters. This approach naturally provides pressure and fluxes continuity along land surface. An advective-diffusive transport equation is incorporated in the model in order to follow and identify pre-event and event water. This multi-domain Richards equation is solved with a mixed hybrid finite element formulation. The time discretization is implicit and the nonlinear equations are solved within a sequential iterative Picard scheme. This model can describe both infiltration excess or saturation excess runoff. In order to assess this modelling approach, several classical validation, verification and application test cases are presented. For overland flow solely, the model is compared to an analytical solution and to commonly used hydrological models. The integrated model is then validated with a sandbox laboratory experiment. Finally, the dynamic response of a saturated contributing area at the toe of a 3D virtual hillslope is simulated to illustrate the applicability of the model to 3D situations. (C) 2008 Elsevier B.V. All rights reserved.